Specific protein-protein interactions are fundamental to most cellular and organismal functions. Polypeptide interactions are involved in formation of functional transcription complexes, signal transduction pathways, cytoskeletal organization (e.g., microtubule polymerization), polypeptide hormone receptor-ligand binding, organization of multi-subunit enzyme complexes, and the like.
Investigation of protein-protein interactions under physiological conditions has been problematic. Considerable effort has been made to identify proteins that bind to proteins of interest. Typically, these interactions have been detected by using co-precipitation experiments in which an antibody to a known protein is mixed with a cell extract and used to precipitate the known protein and any proteins which are stably associated with it. This method has several disadvantages, such as: (1) it only detects proteins which are associated in cell extract conditions rather than under physiological, intracellular conditions, (2) it only detects proteins which bind to the known protein with sufficient strength and stability for efficient co-immunoprecipitation, and (3) it fails to detect associated proteins which are displaced from the known protein upon antibody binding. For these reasons and others, improved methods for identifying proteins which interact with a known protein have been developed.